Before getting into the details of command interpretation, I thought I'd
give a very simple example of why it's important. Here's an error
that occurs all the time. Let's say you have two files, called
file1
and
file2
. You want to create a new version of
file1
that has
file2
added to the end of it. That's what
cat
is all about, so you give the command:

%
cat file1 file2 > file1
...wrong

This looks like it should work. If you've ever tried it, you know it
doesn't; it erases
file1
, and then dumps
file2
into it.
Why?
The shell (not
cat
) handles standard input and output.

As the shell is processing the command, it sees that you're
redirecting standard output into
file1
, so it opens the file for
writing, destroying the data that's already in it.

cat
reads
file2
(which also goes into
file1
). At
this point,
cat
is finished, so it exits.

file1
and
file2
are identical, which isn't what
you wanted. But it's what you got.

Some versions of
cat
give you a warning message in
this situation
(
cat: input file1 is output
). This might lead you to believe that
somehow
cat
was smart and managed to protect you. Sadly, that's
not true. By the time
cat
figures out that an input file and an
output file are the same, it's too late:
file1
is already gone.
This bit of
cat
ty cleverness does have a function, though: it prevents
commands like: